First things first, I do love these deep dives by people like Guilherme Rambo. This is not a leak, but more of a grind-it-out, pay attention to the details analysis.
In this case, Guilherme came upon some code with a log message that said:
“This is where the 3rd party face config bundle generation would happen”
It’s clear from the wording of the message that this feature is not implemented at the moment, but it’s definitely something Apple has planned. This new capability could come as soon as watchOS 5, or be dropped altogether. I personally hope they go forward with it since it’d be pretty cool to be able to install new watch faces on my Apple Watch.
Coincidentally, this past Friday we posted this thread of 3rd party FitBit watch faces. It ain’t pretty.